Techniques of calorimetry are used to study the energy of metabolism in humans and animals. Calorimetry is used, for example, for diagnosis of metabolic disorders and for calculating nutritional requirements of a subject. Calorimetric measurements can be made directly as a measure of heat loss of a subject. Alternatively, indirect measurements can be made of a chemical by-product of metabolism.
A useful measure for nutritionists and sports scientists when assessing the health and fitness of a subject is the volume of oxygen consumed at rest and during or after physical exertion.
Indirect calorimetry often involves measuring the amount of carbon dioxide exhaled by a subject, which can in turn be used to calculate the oxygen consumption of the subject.
U.S. Pat. No. 5,178,155 discloses an indirect calorimeter that comprises a carbon dioxide scrubber arranged to remove carbon dioxide from the exhaled gas and in some embodiments from the inhaled gas also. The volume of oxygen consumed can be calculated from the amount of carbon dioxide removed from the exhaled gas by the scrubber. The calorimeter of U.S. Pat. No. 5,178,155 also comprises two flow meters, the first being arranged to monitor the inhaled gas flow and the exhaled gas flow after removal of the carbon dioxide and the second being arranged to monitor the exhaled gas flow before removal of the carbon dioxide.
International patent application number PCT/US99/17553 discloses a method and apparatus for analysing respiratory gases to determine oxygen consumption for indirect calorimetry purposes as well as carbon dioxide production by measuring the mass and volume flow rate of inhaled and exhaled gas. The flow measurements are made using known ultrasonic pulse transit time techniques. The gas density may be determined using acoustic impedance, speed of sound or temperature related techniques. The method involves calculating the oxygen content in the inhaled and exhaled gas. In one embodiment of PCT/US99/17553 measurements are made on the exhaled gas flow only. In this case, it is necessary to be aware of the oxygen content of the inhaled gas.
International patent application number PCT/US98/05297 discloses a calorimeter comprising a flow meter and a capnometer to compute the difference between the inhaled gas volume and the volume of the exhaled gas less the carbon dioxide volume. The capnometer measures the concentration of exhaled carbon dioxide. The amount of carbon dioxide in the exhaled gas can then be subtracted from the total volume of exhaled gas to obtain a value for the volume of exhaled gas excluding carbon dioxide. A bidirectional flow mater is also used to measure the flow rate of the inhaled and exhaled gas.
International patent application number PCT/US91/04587 discloses a calorimeter comprising means to condition inhaled gas to a temperature and water vapour comparable with that of the exhaled gas, then the volume of inhaled gas is measured. Carbon dioxide is removed from the exhaled gas by passing the exhaled gas through a carbon dioxide scrubber and then its volume is measured. A single flow meter may be used to measure the volume of both the inhaled and exhaled gas.
All of the indirect calorimeters described above are complicated to manufacture and use. It is an aim of preferred embodiments of the present invention to provide an alternative method and apparatus for calculating respiratory oxygen consumption and to provide an alternative indirect calorimeter and method of using the same.